1. Field of the Invention
The present invention relates to an air conditioner capable of running through a year regardless of an outside air temperature, and particularly, to an air conditioner capable of running in forced circulation operation with a compressor run and natural circulation operation with the compressor stopped.
2. Discussion of Background
In recent years, a technical field of removing heat of electronic machines represented by, for example a computer center and a base station (i.e., shelter) accommodating relay electronic machines for mobile communication is rapidly developing in accordance with the spread of mobile communication, such as using a portable telephone. Such locations accommodating the electronic machines have to be subjected to air cooling throughout a year.
In such usage, when an outdoor air temperature is low as in a winter season or a night time, it is possible to cool by air ventilation. However, a device for preventing fog, rain, snow, dust and so on from penetrating thereinto becomes necessary and stable air-cooling cannot be performed because an indoor air temperature varies depending on a variation of the outdoor air temperature. Under such conditions, it is possible to use an air conditioner utilizing natural circulation by which heat can be transferred by a refrigerant from indoors to outdoors by using a difference of temperature between the indoor temperature and the outdoor air temperature. The air conditioner utilizing this natural circulation drastically reduces an annual power consumption in comparison with an air conditioner using the forced circulation by a compressor.
Now, an operational principle of air-cooling by the natural circulation will be described with reference to FIG. 15. FIG. 15 shows a structure of an air conditioner utilizing the natural circulation. In FIG. 15, numerical reference 2 designates a condenser; numerical reference 3 designates an outdoor fan; numerical reference 5 designates an outdoor unit; numerical reference 6 designates a liquid pipe; numerical reference 7 designates an evaporator; numerical reference 8 designates an indoor fan; numerical reference 9 designates an indoor unit provided in a space to be air-conditioned; and numerical reference 10 designates a gas pipe.
When the condenser 2 is arranged at a relatively higher position than the evaporator 7, a liquid refrigerant condensed by the condenser 2 flows into the evaporator 7 after descending through the liquid pipe 6 by gravity. The liquid refrigerant delivered into the evaporator 7 evaporates by receiving a thermal load of the indoor region, for example, a space to be air-conditioned. Thereafter, the liquid refrigerant ascends through the gas pipe 10 to thereby return to the condenser 2, whereby a cycle is formed.
Thus, the air-cooling by the natural circulation utilizes a density variation between a liquid refrigerant and a gas refrigerant derived from an altitudinal difference between the indoor unit 9 and the outdoor unit 5, as driving force for circulating the refrigerant. The natural circulation can be realized in a case that the sum of a pressure loss in a refrigerant path such as the condenser 2, the evaporator 7, the liquid pipe 6, the gas pipe 10, and on-off valves in a refrigerant circuit is equal to a pressure increase in the liquid pipe 6 caused by a height of liquid column.
In FIG. 16, a pressure-enthalpy diagram in a cycle of air-cooling by forced circulation operation utilizing a generally used compressor is shown. In FIG. 16, an abscissa designates an enthalpy and an ordinate designates a pressure. In comparison therewith, a pressure-enthalpy diagram in a cycle of natural circulation operation without using a compressor is shown in FIG. 17. Also in FIG. 17, an abscissa designates an enthalpy and an ordinate designates a pressure. A cycle of air-cooling operation by the forced circulation is performed by a structure that a compressor, a condenser, an expansion valve, and an evaporator are sequentially connected by pipes.
In FIG. 16, numerical reference 34 designates an enthalpy decrease and a pressure drop in the condenser; numerical reference 35 designates a pressure drop by the expansion valve; numerical reference 36 designates an enthalpy increase and a pressure drop in the evaporator; numerical reference 37 designates an enthalpy increase and a pressure rise by the compressor; numerical reference 38 designates a refrigerant pressure corresponding to an indoor temperature; and numerical reference 39 designates a refrigerant pressure corresponding to an outdoor air temperature. An arrow shown in FIG. 16 designates a flow direction of the refrigerant. Further, in FIG. 17, numerical reference 40 designates an enthalpy increase and a pressure drop in the evaporator; numerical reference 41 designates a pressure drop in the gas pipe; numerical reference 42 designates an enthalpy decrease and a pressure drop in the condenser; and numerical reference 43 designates a pressure increase obtained by subtracting the pressure drop in the liquid pipe from the pressure rise by the altitudinal difference in the liquid pipe. In comparing FIG. 16 to FIG. 17, a characteristic that an enthalpy variation in the evaporator and an enthalpy variation in the condenser are substantially equal in the cycle of air-cooling by the natural circulation, not like the cycle of air-cooling by the forced circulation utilizing the compressor, and the flow direction of refrigerant are adverse.
Meanwhile, as an Example of air conditioner utilizing the natural circulation, both of an air-cooling operation by the forced circulation utilizing a compressor (hereinbelow, referred to as forced circulation operation) and an air-cooling operation by the natural circulation (hereinbelow, referred to as natural circulation operation) are used as disclosed in Japanese Unexamined Patent Publication Hei. 9-250779 (JP-A-9-250779). FIG. 18 shows a structure of a conventional air conditioner which can perform both of the forced circulation operation and the natural circulation operation.
In FIG. 18, numerical reference 1 designates a compressor; numerical reference 2 designates a condenser; numerical reference 3 designates an outdoor fan; and numerical reference 6 designates a liquid pipe; numerical reference 7 designates an evaporator; numerical reference 9 designates an indoor unit; numerical reference 10 designates a gas pipe; numerical reference 12 designates a bypass pipe for compressor which is provided for bypassing the compressor 1; numerical reference 14 designates an accumulator; numerical reference 13, 22, 44, and 45 respectively designate an on-off valve; numerical reference 46 designates an expansion valve; and numerical reference 23 designates a bypass pipe for bypassing the expansion valve 46 and the on-off valve 45.
In this air conditioner, there are provide the four on-off valves 13, 44, 22, and 45 for bypassing the compressor 1 and the expansion valve 46. The condenser 2 is arranged at a relatively higher position than the evaporator 7, wherein a cycle of natural circulation operation is realized by opening the on-off valves 44 and 22 and closing the on-off valves 13 and 45 when an indoor temperature is lower than an outdoor air temperature. In other words, a liquid refrigerant condensed by the condenser 2 descends through the liquid pipe 6 by the gravity and flows into the evaporator 7 through the on-off valve 22 in the bypass pipe of the expansion valve. The liquid refrigerant delivered into the evaporator 7 evaporates by receiving a thermal load in the indoor. Thereafter, the refrigerant ascends through the gas pipe 10 and the passing through the on-off valve 44 of the bypass pipe for compressor 12, and returns to the condenser 2, whereby a cycle is formed.
When the indoor temperature is higher than the outdoor air temperature, the on-off valves 13 and 45 are opened and the on-off valves 44 and 22 are closed to run in a cycle of forced circulation by running the compressor 1. In other words, the refrigerant gas in the pipe is adiabatically compressed by the compressor 1 to be in a super heated state, whereby the refrigerant radiates its heat to the outdoor air by the condenser 2 and is liquefied to be thereby changed to a refrigerant liquid. Thereafter, the high pressure refrigerant liquid descends through the liquid pipe 6, passes through the on-off valve 45, and depressurized by the expansion valve 46. Thus the refrigerant liquid is changed to wet-vapor of low-temperature and low-pressure under a condition of gas-liquid mixture. Further, the refrigerant absorbers a heat of evaporation from the evaporator 7 to thereby change to a refrigerant gas. Thereafter, the refrigerant gas returns to the compressor 1 after passing through the gas pipe 10 and the accumulator 14. At this time, an excessive refrigerant for the forced circulation operation is stored in the accumulator.
Thus, in this air conditioner, it is possible to drastically reduce an annual power consumption because the forced circulation operation and the natural circulation operation are switched depending on an outdoor temperature and an indoor temperature and when the natural circulation operation is conducted the driving force becomes only an input to the indoor fan 3. Further, not shown herein, there are many cases that an indoor fan is provided on the side of the indoor unit 9. In such cases of using a unit having both of an outdoor fan and an indoor fan, the annual power consumption can be drastically reduced.
In this, a quantity of refrigerant required for the natural circulation operation is generally greater than that for the forced circulation operation because of a difference in a condition of the refrigerant in the refrigerant circuit. Therefore, the conventional air conditioner had a structure such that the expansion valve 46, which has been used to be provided at around the outlet of the condenser 2, was disposed at the side of indoor unit so that a difference between the quantity of refrigerant under the natural circulation operation and that under the forced circulation operation could be absorbed. Practically, when the forced circulation operation is switched to the natural circulation operation, an excessive refrigerant stored in the accumulator 14 at the time of forced circulation operation should have been collected to send it back to the condenser 2 before the natural circulation operation is performed by a refrigerant recovery operation. Accordingly, in a conventional air conditioner in which forced circulation operation and natural circulation operation were combined had four on-off valves 44, 13, 22, and 45 and pipes for connecting these in order to switch the refrigerant circuit between these operations and recover the refrigerant at the time of switching the operations.
Further, the temperature in a base station accommodating a computer center and a relay electronic machine for mobile communication is controlled in a range of about 25.degree. C. through 35.degree. C. However, when an outdoor air temperature is low as in a winter season or the like, cooling capability obtainable by natural circulation operation is increased, whereby the compressor 1 is in a stopped state for a long time and the temperature of the compressor decreases in accordance with a lapse of time. As the temperature of the compressor 1 decreases, the refrigerant gas is gradually condensed in the compressor 1 by a cycle of the natural circulation operation. Therefore, there was a possibility that not only the quantity of refrigerant necessary for the natural circulation operation was not secured but also a phenomenon of reaching a breakage by a generation of a compression of liquid refrigerant was caused at a time of starting the compressor 1.
In the conventional air conditioner using a combination of forced circulation operation and natural circulation operation, four on-off valves 44, 13, 22, and 45 for switching refrigerant circuits with respect to these types of operation and pipes of connecting these valves for recovering the refrigerant at the time of switching the operations were provided. There was a problem that a system using the combination of the forced circulation operation and the natural circulation operation became costly in comparison with an air conditioner using only a forced circulation because expensive on-off valves having a large inner diameter were used to reduce a pressure loss for the on-off valves 22, 44 provided in refrigerant paths for the natural circulation operation among the above on-off valves. Further, there was a problem that accommodation into an outdoor unit 5 was difficult because the refrigerant circuit was complicated by existence of many on-off valves and the space in the outdoor unit 5 is limited.
Further, at the time of switching to the natural circulation operation, it was necessary to perform refrigerant recovery operation for recovering an excessive refrigerant accumulated in the accumulator 14 at the time of the forced circulation operation on the side of condenser 2. However, when the refrigerant recovery operation was performed by completely closing the expansion valve 46, a suction pressure by the compressor 1 was abruptly reduced, whereby a refrigerant liquid taken in by the compressor 1 was gassed and a refrigerating machine oil flowed out to the refrigerant circuit along with the discharging gas, whereby there was a possibility that seizure was caused by mal-lubrication by the reduced quantity of refrigerating machine oil in the compressor.
Further, the refrigerating machine oil flowed into the refrigerant circuit causing an increment of pressure loss, whereby cooling capability in the natural circulation operation was deteriorated.
Further, when an outdoor temperature was low, such as in a winter season, the cooling capability obtainable by the natural circulation operation was increased, whereby the compressor was stopped for a long time and the temperature of compressor 1 was decreased in accordance with a lapse of time. In such a case, the refrigerant gas was gradually condensed from the natural circulation circuit to the compressor 1, whereby not only the quantity of refrigerant necessary for the natural circulation operation could not be secured but also there was a possibility that breakage occurred by a compression of the liquid refrigerant at the time of starting the compressor 1.
Further, when a flowing direction of the refrigerant in the condenser 2 is upward and when a stand-up pipe vertically existed in connection piping between the outlet of the condenser 2 and the liquid pipe 6, there was a problem that stable cooling capability was not obtainable because the condensed refrigerant liquid was accumulated in a middle of a heat transmission pipe in the condenser 2 or in a middle of a connection pipe and therefore the natural circulation operation became unstable.